Voice telephony systems often provide call coverage for calls that are not answered by a called endpoint or extension. Furthermore, a telephony system will typically permit a sequence of alternate destinations or coverage points to be defined for each extension on the telephony system. The coverage paths are usually stored as instructions in a data base that is referenced when coverage for a call is required.
In a typical private branch exchange system (PBX), a number of communication endpoints, each having a unique extension, are associated with a communication server. Furthermore, a private branch exchange may include a number of subsystems that each include a communication server and a number of extensions. For example, in a typical implementation, a main or central system may be interconnected to one or more remote branch systems by a wide area network (WAN). Furthermore, various coverage operations may be provided to extensions associated with remote branches by the main system server or some other server that is accessed from remote branches over a wide area network connection. Examples of call coverage services provided by a central system include voice mail coverage.
During normal operation, coverage for an extension may be provided by forwarding the call across a local area network from a gateway or communication server that received the call, to the coverage point. In such a situation, the extension of the call communication endpoint is provided as part of the call coverage operation. However, when the gateway or communication server of the system receiving a call is unable to connect to the coverage point, for example due to a WAN outage, the call must be forwarded to the coverage point over an alternate path. Such an alternate path may be provided by the public switched telephony network (PSTN). However, routing a call to a coverage point across the PSTN is typically performed without providing the coverage point with an indication of the extension that was called. Accordingly, upon receiving a call requiring coverage from the PSTN, the coverage point must request information identifying the called extension from the calling party, for example by playing a recorded message requesting that the calling party re-enter the called extension. Furthermore, prior art solutions for implementing coverage paths do not easily accommodate coverage paths that are to be used only under particular circumstances, such as where a branch system is unable to connect to a main or central system over a WAN, and is therefore operating in a local survivable processor mode.
Another problem with distributed PBX systems having separate branches (or a branch and central system) that are connected by an Internet protocol network, such as a wide area network, that becomes unavailable relates to calls between extensions associated with different branches. In such a situation, calls between extensions in different branches (or between a branch and a central system) fail if the caller enters only the extension of the called number. Furthermore, in such situations, the caller will be unable to reach the called extension unless the caller knows the complete telephone number for the called party, in order to allow the call to be routed over the public switched telephony network.
PBX systems that allow coverage arrangements having multiple coverage points, including coverage points accessed across the PSTN, have been available. In order to forward a call across the PSTN for coverage with an identification of the called extension, those systems require that a coverage path including the telephone number of the coverage point and the extension of the called endpoint be coded for each extension. However, providing such a unique coverage path script for each extension in a system having a large number of extensions is impractical. Furthermore, many private branch exchange systems limit the number of coverage paths that may be administered, making the administration of a unique coverage path for each extension impossible. Even if the number of coverage paths that can be defined is unlimited, administering a unique coverage path for each extension would be an administrative burden.